1. Field of the Invention
Aspects of the present invention relate to a water recovery system of a direct liquid feed fuel cell, and more particularly, to a water recovery system for a direct liquid feed fuel cell that mixes water produced at a cathode electrode with a fuel supplied to an anode electrode.
2. Description of the Related Art
A direct liquid feed fuel cell is an apparatus that generates electricity via electrochemical reactions between an organic fuel, such as methanol or ethanol, and oxygen in the air as an oxidant. The electricity generated by the direct liquid feed fuel cell has high specific energy density and high current density. Also, since a liquid fuel, i.e., methanol, etc., is directly fed to the fuel cell, the direct feed fuel cell does not require a peripheral device such as a fuel reformer, and storing and supplying of the fuel are easy and economical.
As depicted in FIG. 1, a unit cell of a direct feed fuel cell has a membrane electrode assembly (MEA) structure having an electrolyte membrane 1 interposed between an anode electrode 2 and a cathode electrode 3. The anode and cathode electrodes 2 and 3 respectively include diffusion layers 22 and 32 for supplying and diffusing a fuel and oxidant, catalyst layers 21 and 31 at which oxidation and reduction reactions of the fuel occur, and electrode supporting layers 23 and 33. Reference numeral 4 indicates conductive plates that include flow channels 41 and 42 through which the fuel supplied to the anode and oxidant supplied to the cathode electrodes 2 and 3 flows, as well as exhaust constituents flowing from the anode and cathode electrodes.
An electrode reaction of a direct liquid feed fuel cell, such as a direct methanol fuel cell (DMFC), which is a direct liquid feed fuel cell that uses a mixture of methanol and water, includes an anode reaction where fuel is oxidized and a cathode reaction where hydrogen and oxygen are reduced as described below.CH3OH+H2O→CO2+6H++6e− (Anode reaction)  Reaction 13/2O2+6H++6e−→3H2O (Cathode reaction)  Reaction 2CH3OH+3/2O2→2H2O+CO2 (Overall reaction)  Reaction 3
At the anode electrode 2 where the fuel is oxidized (reaction 1), one carbon dioxide molecule, six hydrogen ions, and six electrons are produced for each methanol molecule consumed. The produced hydrogen ions migrate to the cathode electrode 3 through the electrolyte membrane 1. At the cathode electrode 3, water is produced by the reduction reaction (reaction 2) between hydrogen ions, electrons transferred through an external circuit, and oxygen. Accordingly, in the overall electrochemical reaction of the DMFC, water and carbon dioxide are produced by the reaction between methanol and oxygen (reaction 3).
The theoretical voltage that can be generated by a unit cell of a DMFC is approximately 1.2V. However, an open circuit voltage at an ambient temperature and at atmospheric pressure falls below 1V due to a voltage drop caused by an active surcharge and a resistance surcharge. In reality, an actual operating voltage lies in the range of 0.3˜0.7V. Therefore, to obtain a higher voltage, a plurality of unit cells must be connected in series.
When a high concentration fuel is used, there is a large reduction of electricity output due to cross-over (a phenomenon where fuel passes through an ion exchange membrane) of the fuel through the electrolyte membrane (i.e., a hydrogen ion exchange membrane). Therefore, in a fuel cell, a liquid fuel mixed with water produced in the system or already stored in a water storage rather than a pure liquid fuel is used. However, when a low concentration fuel is used, the volume of a fuel tank must be large. A large fuel tank can hardly be applied to a small fuel cell system.
A mobile small fuel cell can be a monopolar type direct liquid feed fuel cell having a plurality of unit cells. However, to generate high density electricity, a high concentration fuel, for example, pure methanol can be used, and a water recovery system to recover water from a cathode electrode and to supply a dilute methanol to an anode electrode is required.